The only It girls on the list now that i think of are:
Britney: She caused commotion in the 90s with the BOMT era.
Miley Cyrus: There was even Hannah toillet paper i think
And all of her albums had go #01
Avril Lavigne: Every girl dressed like her, every teenage knew Complicated.
Hilary Duff: The whole world knew Come clean and So yesterday. And she had the most popular Disney series.
Popular Girls:
Christina, Ashlee, Jennifer, P!nk, Rihanna and Beyonce.
Gaga, Taylor and other girls that are on the list can be considered NEW IT GIRLS since they are new artist.